In systems using an optical pen and a handwriting input sheet for digitizing handwritten information such as characters or figures and inputting these into an information processor, the handwriting input sheet usually include a layer in which information is written as an optical pattern and a reflection film for returning light emitted from an optical pen to an imaging element built in the optical pen as light reflecting the information described as the optical pattern. As an example, in Patent Literature 1, there is a description regarding an information input auxiliary sheet, in which an infrared reflecting layer having characteristics of reflecting infrared rays from one face side and allowing visible light to pass through is provided together with a dot pattern layer in which dots of a dot pattern defining repeatedly coordinate information and/or code information are arranged.